The present invention relates to a multifocal spectacle lens having a single-part base lens member and at least two adjacent additional parts which are limited to a region of smaller diameter than the single-part base lens.
If such a spectacle lens consists of glass, it is generally produced by fusing the additional lenses into one of the surfaces of the base lens. For this purpose, there is customarily first produced a button-shaped part which consists of three adjoining strips which are fused together. The upper strip consists of glass having the same index of refraction as the base glass, the middle strip consists of glass of higher index of refraction, and the lower strip consists of glass of still higher index of refraction.
After the working of the fusion surface having the radius R.sub.3 this button is then fused by known methods into the base glass. Thereupon, the base surface of the spectacle lens is cut and polished. It has the radius R.sub.1 or R.sub.2 depending on whether the fusion lies in the outer or inner surface of the base glass. In this way, there is produced, as intersection line of the surface having a radius R.sub.1 or R.sub.2 respectively with the fusion surface of radius R.sub.3, a circular limitation of the lower addition part, the center of which lies on or in close proximity to the lower separation line between upper and lower addition parts. At this place the tangential planes to the surfaces having the radius R.sub.3 and R.sub.1 or R.sub.2 respectively are parallel, so that no image jump occurs upon passage (in a vertical direction) of the viewing rays from one addition part to the other.
On the other hand, in this conventional known construction an image jump does occur at the upper separation line between base glass and upper addition part, since the tangential planes to the surfaces having the radius R.sub.3 and R.sub.1 or R.sub.2 respectively form an angle with each other also in the center of the separation line. This image jump is a functional defect. Since the separation line between upper and lower addition parts passes through the center of the circle of intersection, the corresponding separation surface passes through the thickest point of the fused-in lens and is therefore particularly wide and particularly disturbing.
A trifocal lens which is free of image jump is also known in British Pat. No. 951,524, in which lens a semicircular addition lens is fused into a first base part and this base part is fused or cemented over its entire outer surface to a second base part the inner surface of which contains an addition lens which is also semicircular. The addition lens in the second base part is smaller than the one in the first base part. Aside from the fact that this solution cannot be esthetically satisfying, the following functional defects result:
1. The lens surface is cut up by a large number of boundary lines which impede vision.
2. Since the smaller lens must have a minimum height of 12 mm and the intermediate part of a height of 7 mm, a radius of at least 19 mm results for the larger additional lens. Since the larger additional lens thus has a diameter of at least 38 mm it is necessarily thicker and therefore has a very wide separation surface.
3. In addition to the thick larger additional lens there is not only the smaller thickness of the smaller additional lens but also two tolerance spacings between the fused-in surfaces and the outer surfaces of the corresponding base part, so that the spectacle lens becomes thick and heavy.